Known hydraulic drive devices of the above type are proposed by, for example, patent documents 1, 2. The hydraulic drive device of patent document 1 is configured to set an engine output torque characteristic and a hydraulic pump absorption torque characteristic that are suitable for a working mode selected by an operator. The hydraulic drive device of patent document 2 is configured to detect a certain operation status of, for example, an implement through detection of a certain operation status of an operating lever and set a maximum value of absorption torque of a hydraulic pump at a predetermined value in accordance with the implement's certain operation status when the implement is in that operation status.
Here, patent document 1 is Japanese Patent Unexamined Publication No. H02-38630, and patent document 2 is Japanese Patent Unexamined Publication No. 2002-295408.
In the hydraulic drive device of patent document 1, when a heavy excavation mode is selected by the operator, an engine is operated at full load, and the engine output torque characteristic indicated by line ELa in FIG. 11(a) is set. This engine output torque characteristic line ELa has regulation line Ra set for set engine speed NA and is set such that output (horsepower) of the engine reaches a maximum at engine speed Na. Output torque Ta of the engine corresponds to this engine speed Na (Hereinafter, an output torque point specified by engine speed Na and engine output torque Ta is referred to as “output torque point Ma”). The hydraulic pump absorption torque characteristic indicated by line PLa in FIG. 11(a) is also set through adjustment of the amount of oil discharged from a variable displacement hydraulic pump (hereinafter simply referred to as “hydraulic pump”). This hydraulic pump absorption torque characteristic line PLa is set to be a monotone increasing function having engine speed as a variable. At output torque point Ma, the output torque of the engine is matched with the absorption torque of the hydraulic pump (Hereinafter, such a condition is referred to as “matching”). This means that the hydraulic pump absorbs the engine horsepower at output torque point Ma, i.e., the maximum horsepower of the engine, thereby allowing high-efficiency heavy excavation work. A method of, for example, setting engine output torque Ta and engine speed Na, which specify output torque point Ma, as target values as described above and increasing and reducing the absorption torque of the hydraulic pump based on calculation of a difference between the target engine speed and an actual engine speed for matching the absorption torque of the hydraulic pump with the engine output torque at output torque point Ma is called “engine speed sensing control” and is a publicly known technique.
When an excavation mode or a leveling mode is selected by the operator, the engine is operated at partial load, and the engine output torque characteristic indicated by line ELc in FIG. 11(b) is set in the above-mentioned hydraulic drive device. Engine output torque characteristic line ELc has regulation line Rc set for set engine speed NC. The hydraulic pump absorption torque characteristic indicated by line PLc in FIG. 11(b) is also set through the adjustment of the amount of oil discharged from the hydraulic pump. Here, the absorption torque of the hydraulic pump is controlled at values along an equivalent horsepower curve of the engine according to the set engine speed. A method of matching such absorption torque of the hydraulic pump with the output torque of the engine is called “equivalent horsepower control”. In the case of FIG. 11(b), matching is done between the output torque of the engine and the absorption torque of the hydraulic pump at output torque point Mc specified by engine speed Nc and engine output torque Tc corresponding to engine speed Nc (Hereinafter, output torque point Mc is referred to as “matching point Mc”). This case allows the matching between the engine output torque and the absorption torque of the hydraulic pump at matching point Mc which has a lower fuel consumption rate (g/kw·h) than that of matching point Mc′ which is specified when the set engine speed is NA, so that the engine is used within a range of better fuel efficiency while maintaining the same horsepower.
In the hydraulic drive device of patent document 2, when, for example, the certain operation status of the operating lever is detected with the heavy excavation mode selected, the hydraulic pump absorption torque characteristic indicated by line PLd in FIG. 11(c) is set through the adjustment of the amount of oil discharged from the hydraulic pump. Here, the absorption torque of the hydraulic pump is kept constant. A method of matching such absorption torque of the hydraulic pump with the output torque of the engine is called “constant torque control”. In the case of FIG. 11(c), the matching is done between the output torque of the engine and the absorption torque of the hydraulic pump at output torque point Md specified by engine speed Nd and engine output torque Td corresponding to engine speed Nd (Hereinafter, output torque point Md is referred to as “matching point Md”). Setting the maximum value of the absorption torque of the hydraulic pump at matching point Md in accordance with the implement's certain operation status when the implement is in that operation status limits the hydraulic pump to an output required for the operation, thus reducing fuel consumption at the time of light-load operation.